Joe Hockey's first budget, which delivered swingeing cuts to health, education and welfare, raised taxes and broke a series of promises, was ''much softer'' than the Treasurer had wanted, a new biography published on Wednesday says.

Mr Hockey was convinced by Prime Minister Tony Abbott to take a more cautious approach, delaying cuts to pensions and reducing the number of people who would pay the deficit levy during meetings of the expenditure review committee.

The Treasurer is also named as Mr Abbott's most likely successor by the Prime Minister's chief of staff, Peta Credlin, who declares Mr Hockey has ''got his head above every other contender'', though she cautions he is not yet the ''heir apparent'' to replace Mr Abbott.

Related Content

The revelations are contained in Hockey: Not Your Average Joe, written by Fairfax Media columnist and long-time ABC broadcaster Madonna King.

The budget cut a range of entitlements for older people, including slowing the indexation of pension growth, and imposed a 2 per cent deficit levy on incomes over $180,000 - at one point, the government is said to have considered imposing the levy on incomes greater than $80,000.

The book also details tensions between Mr Hockey and Industry Minister Ian Macfarlane over the federal government's tough stance on industry assistance to General Motors Holden as well as revealing an unlikely friendship between the Treasurer and former Labor prime minister Bob Hawke.

The book quotes a source inside the room during meetings leading up to the budget as saying: ''It wasn't as tough as Joe would have liked but a good compromise. Maybe it's tougher than the Prime Minister would do if Joe wasn't there to drive it.''

King, who had Mr Hockey's co-operation for the book, goes on to say: ''In reality, the budget was much softer than Joe would have liked. He wanted changes to pensions made earlier and the deficit levy to net more taxpayers … But Abbott, who chaired each of the expenditure review committee meetings, was taking a much more cautious approach than his Treasurer, no doubt with one eye firmly on the reaction of voters.''

Ms Credlin's comments are likely to raise eyebrows in Coalition ranks as it is highly unusual for political aides to comment publicly.

Mr Hockey's chief of staff, Grant Lovett, is also quoted in the book as are many of Mr Hockey's former staff.

Former prime minister John Howard, former treasurer Peter Costello, Treasury secretary Martin Parkinson and many of Mr Hockey's family and colleagues co-operated with the book.

The federal government has suffered a dramatic slump in its standing with voters since the budget, which has been widely viewed as unfair and too tough.

The Coalition's two-party preferred vote has plummeted from a 51-49 lead over Labor in a Fairfax/ Nielsen poll published in March to trail 44-56 in May, days after the budget was released.

A Fairfax/Nielsen poll published on Monday confirmed that the Coalition trailed Labor 46-54, while Mr Hockey's personal standing had slumped dramatically since the federal budget.

In March, he led Labor's Chris Bowen as preferred Treasurer by 51 to 34.

However, this now stands at 43-42, giving Mr Hockey just a one-point lead.